Modern hemodialysis (HD) machines are able to measure ionic dialysance online and thereby continuously monitor Kt/V. The accuracy of measurement depends on the input of the correct urea distribution volume (V), available from anthropometric equations and body composition monitoring (BCM). The latter method, however, has not been validated in children.We compared V determined by BCM to that calculated using four different anthropometric formulas (Morgenstern, Mellits and Cheek, Hume-Weyers and Watson equations) in 23 pediatric HD patients. We also compared online Kt/V using BCM-derived V with the Kt/V calculated from pre- and post-dialytic urea concentrations using the single-pool second-generation Daugirdas equation.The V calculated by the Morgenstern equation was similar to that derived by BCM, with a mean difference of -0.7% (95% limits of agreement -11.7 to 10.3%). In contrast, the V calculated by the other equations was 5.4, 6.2 and 18%, respectively higher than the BCM-derived V. The mean difference between Kt/V calculated using the Daugirdas equation and online Kt/V determination based on BCM-derived V data was 0.10 (95% limits of agreement -0.50 to 0.70%).In our pediatric HD patients the V measured by BCM was in agreement with that calculated using the Morgenstern equation, which is the only equation which has been validated to date in children on dialysis. Online determination of Kt/V using a BCM-derived V largely agreed with that calculated by the Daugirdas equation. We therefore conclude that the former approach is suitable for frequent online assessment of dialytic small solute clearance.